1. Field of Endeavor
The present invention relates to a component, especially a hot gas component of a turbomachine, with at least one passage which is embedded in an outer wall of the component and basically extends parallel to and close to the surface of the component, which passage is especially to serve for intake of a cooling medium.
Furthermore, the invention relates to a method for producing at least one such passage in a component.
2. Brief Description of the Related Art
Modern turbomachines, like, for example, gas turbines, are exposed to high loads during operation. They are often operated with hot gases at more than 800° C., and at the same time are subjected to high mechanical loads. The increase of turbine output capacity during the last decades is fundamentally based on two improvements. On one hand, continuously new efficient materials were developed, like, for example, monocrystal alloys, as a result of which the load capacity of the components which lie in the flow cross section of the hot gases could be increased, and on the other hand, cooling systems and temperature protective coatings, which were improved time and again, were developed, which resulted in an increase of the turbine inlet temperatures, and, as a result, an increase of the turbine output. Monocrystal alloys, like, for example, CMSX2, CMSX4, or MK4, have especially led to an appreciable reduction of the temperature sensitivity, and, as a result, to appreciably improved mechanical properties at high temperatures. Since the output capacity of gas turbines is directly coupled with the inlet temperature of the hot gases, for years a continuous increase of the hot gas temperature has been noted, so that especially in the first turbine stages gas temperatures are already achieved which exceed the melting temperatures of the alloys which are used there. In order to prevent damage of the hot gas components or the alloys which are used, as the case may be, complex internal cooling systems were developed, which cool the components which lie in the flow cross section in such a way that these lie below a critical temperature limit at which the components would be damaged. In this case, it is common to all cooling systems that a compromise has to be made between the desired cooling effect, the amount of cooling air which is available, and the costs.
The cooling air which is required for cooling is generally delivered from a compressor and, by means of an internal cooling system, is distributed to the components which are to be cooled.
As a rule, different cooling methods, like, for example, such methods which are based on a convective heat transfer, are combined with a film cooling or transpiration cooling. In this case, the components have internal cooling passages, for example, which extend in serpentine-like fashion and which interact in a communicating manner with a multiplicity of discharge openings on a surface of the component, as a result of which a film cooling or transpiration cooling is created. An especially effective cooling is achieved in this case, if the wall which is to be cooled has a wall thickness which is as small as possible (EP 0 964 981).
Calculations have proved that in a development of a cooling method of a hot gas component to the effect that a system of cooling passages which is close to the surface is created, which cooling passages communicate at least by their one end with the internal cooling medium passages which pass through the inside of the blade mostly in serpentine form, while at least one other end is connected to cooling paths which lead to the surface and effect a film cooling or transpiration cooling there, lead to an increase of the turbine inlet temperature by 50K to 125K, and lead to a significant enhancement of the machine output as a result of it, without additional consumption of cooling air.
Since, however, as a result of the cooling of the components, especially of the turbine blades, the overall efficiency of the power plant decreases, a compromise also has to be found here between turbine output and turbine cooling.
Another efficient, convective cooling system is effected by means of coolable wall structures, as it is proposed, for example, in EP 1 462 611, EP 1 462 612, and EP 1 462 613. In this case, the walls of the hot gas components are equipped with a network of cooling passages. In the interests of effective cooling, it is advantageous to construct these walls very thinly and to lay out the cooling passages close to the thermally stressed surface. In this way, an efficient cooling can be provided. However, such internal cooling passages are exceptionally complicated to produce in the manufacturing process and, therefore, are disproportionately expensive.
To alleviate this disadvantage, a method for producing or repairing cooling passages, which are close to the surface, in a hot gas component of the gas turbine has become known, which is basically based on a profile being applied to a basic body of this component, which profile corresponds to the later structure of the cooling passages. This can be carried out either in the way by a thermally stable filling material in a corresponding structure being applied to the surface of the basic body, or by this structure first being mechanically machined from out of the surface of the basic body and the cavities which result from it then being filled with the thermally stable filling material. In a subsequent step, a coating material is applied by means of a coating method, at least in the region of the cooling structure. The cooling passages are opened by means of subsequent removal of the filling material.
This proposal which forms a generic type is disclosed in EP 1 065 026 and also in later publications, like EP 1 462 611 and EP 1 462 612.
By means of these solutions, it will be possible on one hand to create a cooling passage network in a component of a turbomachine, which on one hand brings about an efficient cooling of the component on account of its position which is arranged just beneath the surface of the outer wall, and which on the other hand can dispense with costly casting molds and results in lower scrap rates. The cooling passages, which are embedded in the outer wall of the component, can generally also be combined with other cooling strategies, like, for example, the transpiration cooling which is described above, as a result of which a high flexibility and a broadened application spectrum can be achieved.
In this method, however, the relatively high manufacturing cost is still disadvantageous, especially the regular requirement of an aftertreatment for the subsequent removal of the filling material.